The present invention relates to a display processor for producing video signals from digitally encoded data and, more particularly, relates to a display processor for word processing applications wherein video signals are produced to create an alphanumeric display that emulates a full page of typewritten copy.
Computer driven displays are found in a variety of business and scientific areas for information retrieval. Efforts to expand the application of digital technology into new areas have brought the computer driven display into use in many new applications. Most recently, word processor systems utilized for document creation and text editing have become another new application for computer driven displays.
In display based word processor systems, text is digitally encoded and stored to facilitate editing, retrieval and output. A typewriter based editor performs editing operations directly on the storage media and a display emulates the typewritten copy as it will actually appear upon print out. With a display based word processor, a visual representation is produced directly from the storage media, with the image instantaneously reflecting keyboard and editing actions.
Display based word processor systems utilized for document generation and text editing wherein a display provides a full-page alphanumeric display that emulates paper copy have generally utilized a raster scan cathode ray tube (CRT). Although other display technology, such as light emitting diodes and liquid crystal displays, have been proposed, the cathode ray tube has been the dominant display technology.
Emulation of the typewritten page necessitates that the display processor accommodate a variety of character and text line spacings. Also, it is desirable to provide for superscript/subscript and underlining. Features, such as field blinking and screen inversion to focus attention to various areas of the display, are also considered to be of value.
A display based word processor system designed for providing representations of a full page of typewritten copy has as the basic input/output devices a typewriter style keyboard, a CRT display, a printer and a storage media. The keyboard and the image produced by the full-page alphanumeric display serve as the primary interface for operator/machine dialogue.
A display processor is utilized to transform digitally encoded representations of alphanumeric characters into the necessary video drive signals for the CRT display. The display processor receives the digitally encoded representations via a direct memory access bus line. Typically, the text data is encoded in the ASCII format. The drive signals required are the serial video data and horizontal and vertical sync. The alphanumeric characters displayed on the CRT are made up of a dot matrix that is displayed in a series of scan lines, with a defined number of scan lines forming a document text line.